The present invention relates to a transmission system using optical fibers and, more particularly, to a long-distance, large-capacity optical transmission system employing return-to-zero lightwave pulses, such as soliton lightwave pulses, and optical amplifiers.
Buttressed by developments of optical amplifying techniques, optical fiber communication technology has made rapid-paced progress toward ultra-long-distance communication, now allowing implementation of a transpacific communication system without the need of using regenerative repeaters. At increased transmission rate, however, conventional transmission systems suffer serious degradation of their transmission characteristices that are caused by the wavelength dispersion characteristic and nonlinear effect of optical fibers, imposing severe limitations on the realization of a high-speed, high-capacity transmission system.
In recent years, an optical soliton communication system has been in the limelight as a system will break the bottleneck in the speedup of transmission by the wavelength dispersion characteristic and the nonlinear effect.
The optical soliton communication system is a system that positively utilizes the wavelength dispersion characteristic and nonlinear effect of optical fibers which are major factors to the degradation of characteristics of the conventional transmission systems and that transmits optical short pulses intact by balancing optical pulse width expansion owing to the wavelength dispersion by the optical fibers and pulse width compression based on the nonlinear effect.
It is in the last several years in which an erbium-doped optical fiber amplifier (EDFA) has emerged as a practical optical amplifier that the long-distance optical soliton communication system has attracted attention as a communication system which has the potential for dramatically increasing the transmission capacity of the optical transmission system. The soliton lightwave pulse, which stably propagates without changing its shape and level, is based on the assumption that it is free from losses of the transmission medium. However, ordinary optical fibers produce losses, and the light intensity becomes attenuated with distance, resulting in the nonliner optical effect being lessened and becoming unbalanced with the dispersion effect. To realize fiber-optic transmission with apparently no loss while keeping the light intensity at a certain value, it is therefore necessary that losses of optical fibers be compensated for by optical amplifiers. In the case of using optical amplifiers as repeaters, it is possible to accomplish soliton communication with practically no waveform variations of lightwave pulses like ideal soliton pulses, by setting the average power between repeaters and the average dispersion of optical fibers to soliton conditions. In the optical soliton communication, optical amplifier noise affects the timing jitter of lightwave pulses at the receiving end and eventually deteriorates the transmission characteristic. That is to say, soliton lightwave pulses with noise superimposed thereon undergo random fluctuations of their light intensity and slightly shift in shape from ideal soliton lightwave pulse, causing fluctuations in the shift amount of the carrier frequency by the nonlinear optical effect. Since this is repeated for each repeater, the time of arrival of lightwave pulses randomly fluctuates during their propagation in optical fibers each having a limited dispersion value, incurring the timing jitter at the receiving end. This phenomenon is called as the Gordon-Haus effect, which is a major limiting factor to the transission characteristic of the optical soliton communication.
There have been intensitively studied soliton control techniques for suppressing the above-mentioned timing jitter, and soliton transmission experiments have made rapid progress in the last few years. One possible method that has been proposed to suppress the timing jitter is to derectly suppress it in the time domain by inserting in the transmission line optical modulators synchronized with transmission signals. This method necessitates, however, the provision of a fast optical modulator and its driver in each optical repeater. This imposes some limitations on ultrafast transmission and makes it difficult to apply this method to submarine cables which are required to be highly reliable. Another method is to insert optical filters in the transmission line to control the random frequency shift in the frequency domain. This method, however, involves the use of optical repeaters each having a fast optical modulator and a driver therefor, and hence poses limitations on the realization of an ultrafast transmission system; besides, this method is difficult to apply to submarine cables which are required to be highly reliable. Another method is to insert optical filters in the transmission line to control the random frequency shift in the frequency domain. This method is easy of use for the speedup of transmission, since the repeater needs only to have an optical filter which is a passive element.
The optical filter itself had already been used in experiments on the soliton transmission (E. Yamada, K. Suzuki, and M. Nakazawa, Electronics Letters, Vol. 27, No. 14, pp. 1289-1291, 4th Jul. 1991), subsequently, it was reported that an optical band-pass filter has the effect of suppressing the timing jitter by bringing close to its center frequency the signal frequency with random shift which is the cause of the timing jitter (A. Mecozzi, et al., Optics Letters, Vol. 16, pp. 1841-1843, December 1991, and Y. Kodama and A. Hasegawa, Optics Lettes, Vol. 17, pp. 31-33, January 1992, and Japanese Pat. Laid-Open No. 227105/93).
This filter is commonly referred to as a frequency guiding filter since it guides in the frequency domain the soliton lightwave pulses which tend to deviate from its center frequency.
Conventionally, there has been used a narrow-band filter of a band about 10 times the full width at half maximum B.sub.sol of the soliton spectrum with a view to effectively suppressing the timing jitter (L. F. Mollenauer et al., Electronics Letters, Vol. 27, No. 22, pp. 2055-2056, 24th Oct. 1991, and L. F. Mollenauer et al., Electronics Letters, Vol. 28, No. 8, pp. 792-794, 9th Apr. 1992). The narrower the full width at half maximum of the band is, the more the guiding effect increases, but the narrow-band filter cuts the lower portion of the soliton spectrum; it is reported that the compensation for the cut portion would apparently give rise to an excessive amplification of a non-soliton component in the vicinity of the center frequency of the filter, making soliton pulses to be unstable. For this reason, it has been considered preferable that the band of the optical filter be around 10 times the full width at half maximum of the soliton spectrum. With such a conventional frequency guiding filter, however, the timing jitter can be suppressed only to a limited extent; in a transpacific communication system using one wavelength (about 9000 km long), the transmission rate is approximately 7.5 Gb/s at maximum and cannot be increased up to 10 Gb/s (L. F. Mollenauer et al., Electronics Letters, Vol. 28, No. 8, pp. 792-794, 9th Apr. 1992).
FIG. 6 shows the results of tests made by the inventors of this application at a transmission rate of 20 Gb/s. In FIG. 6 the ordinate represents the bit error rate and the abscissa the distance of transmission. The bit error rate for a transmission distance 4500 km was 10.sup.-12 and a 9000 km transmission was quite impossible.
To solve this problem, there has also been studied a method of slightly sliding the center frequency of the optical filter with distance, and the filter is called a sliding-frequency guiding filter. The soliton component, which is a nonlinear wave that propagates while making a frequency chirp, follows even slight variation in the center frequency of the filter, but a noise component which is a linear wave does not follow the frequency shift of the filter and is gradually driven out of the band of the filter, with the result that the accumulation of noise is suppressed. The shift amount of the center frequency is about 6 to 7 GHz for 1000 km. There have been conducted experiments on a 10 Gb/s two-wavelength multiplex transmission using the sliding-frequency guiding filter; it has been reported that the use of such a filter would permit transmission for more than 9000 km (L. L. Mollenauer et al., Electronics Letters, Vol. 29, No. 10, pp. 910-911, 13th May 1993).
It is extremely difficult, however, to apply such narrow-band optical filters to actual systems. The frequency of light is approximately 200 THz; for example, in the case of the sliding-frequency guiding filter, the absolute value of its center frequency needs to be shifted around 200 MHz (accuracy of 0.0001% with respect to the center frequency) for each of repeaters installed at intervals of about 30 km. From the viewpoints of the existing technical level and such an environment change as a temperature change of the actual system, it is expected that such precise control of the narrow-band optical filter is almost impossible in the actual system. Taking into account the application of the optical soliton communication to the optical submarine cable, the prior art which involves the use of an ultra-narrow band optical filter in the repeater required to be of high reliability is not desirable from the practical viewpoint such as the long-term reliability of the system.
As described above, it is known in the art that a high-capacity, long-distance optical communication system using the narrow-band filter could be constructed through utilization of the sliding-frequency guiding filter scheme, but as regards optical band-pass filters which is free from the frequency sliding requirement and has a relatively large full width at half maximum of the band, there has been reported only an experiment using a second-order Butterworth filter (M. Suzuki et al., Electronics Letters, Vol. 30, pp. 1083-1084, 23rd Jun. 1995). Since the second-order Butterworth optical filter has a maximum flat amplitude characteristic, its amplitude characteristic is relatively flat and the problem of an excessive gain for linear waves near the center frequency is difficult to arise, but since its group delay characteristic is not flat, the soliton time-waveform is modified. This poses a problem that the full width at half maximum of the band of the filter which can be used in practice is limited to a very narrow range.
As will be appreciated from the above, the conventional optical band-pass filters which suppress the timing jitter have disadvantages such as poor jitter suppressing ability and difficulty in their precise control.